Variable shunt permanent magnet



Ap 1952 H- w. WASHBURN VARIABLE SHUNT PERMANENT MAGNET Filed Oct. 16,1948 AMPLIFIER FIG. 3.

IN VEN TOR. HAROLD W WASHBURN Z A T TORNEY Patented Apr. 22, 1952VARIABLE SHUNT PERMANENT MAGNET Harold W. Washburn, Pasadena, Calif.,assigncr to Consolidated Engineering Corporation, Pasadena, Calif., acorporation of California Application October 16, 1948, Serial No.54,957

Claims. 1

This invention relates to a variable shunt permanent magnet and moreparticularly to the use p of such variable shunt magnet in massspectrometry.

A mass spectrometer is an analytical apparatus which functions to sortand measure ions. Ordinarily it includes an ionization chamber intowhich molecules of the sample to be analyzed are introduced. In theionization chamber the molecules are bombarded by a stream of electronsand converted into ions. So-called propelling or accelerating electrodespropel the ions from the ionization chamber into and through an analyzerchamber. During passage through the analyzer chamber the ions aresubjected to a transverse electric or magnetic field or both to separatethem according to their mass-to-charge ratios into a plurality ofdiverging ion beams. Each ion beam is composed of ions of the samespecific mass and difiering from the mass of the ions forming otherbeams. The diverging beams are successively focused on an ion collectorby varying the potential applied to the accelerating electrode or byvarying the intensity of the transverse field in the analyzer tube. Thecurrent produced by the discharge of each beam on the ion collector is ameasure of the partial pressure of the molecules (from which the ionswere derived) in the sample being analyzed.

As indicated above, the diverging ion beams may be focused on the ioncollector by varying the potential applied to the acceleratingelectrodes or by varying the strength of a transverse magnetic field.The focusing of successive beams on the ion collector is commonlyreferred to as scanning. The advantages of using magnetic scanning, i.e. slow variation in the strength of the transverse magnetic fieldrather than electric scanning, i. e. variation in the potential appliedto accelerating electrodes, may be listed as follows:

1. The mass spectra obtained with magnetic scanning are more constantthan the mass spectra obtained with electric scanning. This is due tothe following facts:

a. When magnetic scanning is used the shift in a position of theelectron beam is less than when electric scanning is used.

I). When electric scanning is used any variation with time of spacecharges or surface charges will cause variations in the spectra. Theeffect of the space charges or surface charges is less apparent whenmagnetic scanning is used and consequently the variations resultingtherefrom are minimized.

c. The voltage effect, about which very little is known,has less effecton magnetic scan than on electric scan.

2. A wider range of masses may be covered in a single sweep whenmagnetic scanning is employed.

Further, there are two methods of scanning by means of the magneticfield. One such method involves gradually varying the current suppliedthrough an electro magnet by means of which the magnetic field isestablished.

Another means described in the present in- I vention comprises graduallyvarying the effective strength of the field established by a permanentmagnet. I accomplish this by adjustment of the mechanical position of apermeable shunt across the gap of a permanent magnet in which gap theso-called analyzer tube or chamber of the mass spectrometer is disposed.A permanent magnet with a variable shunt as described and illus tratedherein has several advantages over an electro-magnet provided with meansfor varying the current. These advantages include:

,1. The permanent magnet can be made in almost any shape desired. In itsapplication to mass spectrometry this permits a reduction of the totalarea of the magnetic field to a minimum whichin turn makes possible asmall coinpact unit.

2. When using a mechanical shunt a power supply and current controlcircuit are eliminated.

, In general, mechanical devices are simpler to maintain and service,particularly when the personnel using the instruments are not trained inelectronics.

3. The mechanical motion employed to move the shunt can be simplytransferred to a calibrated dial which will read the mass directly. Themechanical shunt arrangement therefore makes possible a simple massmarker.

The present invention is directed to a variable shunt permanent magnethaving spaced opposite poles defining an air gap in which a magneticfield is established, and a magnetically permeable member associatedwith the poles and movable adjacent the air gap to shunt a portion ofthe magnetic flux through the member. In a preferred embodiment two suchpermeable shunts are provided; one in association with each poledefining the air gap. Mechanical means are provided for moving the twoshunts simultaneously so that their displacement is equal and opposite.As the shunts are brought closer together a greater proportion of themagnetic flux is by-passed through the shunts thus weakening the fieldstrength in the air gap.

One obvious application of such a variable means for producing amagnetic field across the analyzer tube transverse to the path of iontravel the improvement comprising a permanent magnet having its polesarranged on opposite sides of the analyzer tube vso that the analyzertube lies in an air gap between the poles, and

means for varying the strength of the magnetic field across the air gap.

The means for varying the strength of the magnetic field across the airgap comprises, in accordance with the foregoing description, a magneticshunt disposed adjacent one pole of the magnet and adjustablebymechanical means to reduce or increase the distance between the shuntand the opposite pole. As the name im-- plies, the shuntacts as aby-pass through which the magnetic flux' tends ,'to travel between thetwo poles when the reluctance of the, path through the shunt is lessthan the reluctance of the air gap. As the distance between the shuntand the opposite pole decreases the field strength across the air gap isreduced and as the distance between the shunt and the opposite poleincreases the field strength across the air gap is increased. 1 r r Aparticular feature of, the invention is that the shunt is preferablyconstructed in two parts, each part being in contact with one of themagnetic poles. 'Ihetw'o' parts of, the shunt 'are mounted on. the sameshafts and the motion of the shunts toward or away from the opposingpole pieces is made equal and opposite by the use of the left and righthand thread'sori the, shafts. Thus as the several shafts are turned thetwo -shunts, each contacting a separatefpele piece,j'are forcedclosertogether so as to decrease the reluctance of the shunt circuit and thusreduce the field strength across the air gap. Conversely, as theseveralshafts are turned in the opposite direction the two shunts "areseparated so as to correspondingly increase the romance .or thecircuit'aildfthe field strength cross the all gaps nce the fo ces,parallel jto 'thes'haft or [sh-art's, which are due to the actionoi themagnetic field on the shunts,areapproximatelyequal and opposite theseforces merelyeaujse longitudinal stresses in the shaft and exertpractically no resultant force oflnthe end bearing. 4

The invention will be. more clearly understood by. reference to thefollowing detailed description thereof taken with relation to 'the'accompanying drawings in which: j

. Fig. 1 is a 'p1an view taken'on'theline l-l os Fig. 3 showing,diagrammatically a part of a mass spectrometer and its relation to onepole "of the permanent magnet.

Fig. 2 is an elevation of a variable'shunt permanent magnetsuch as isvemployed with the'ion source and analyzer tube shown'in Fig. 1.

Fig.3 is a sectional elevation taken on the line 33 of Fig. 2 with themass spectrometer analyzer tubeand ion source shownin phantom. Fig. 4 isa plan view taken on'the line 4- 4 of d In the drawings, the variableshuntpermanent magnet of theinvention islsliewn as een'striict'ed foruse in a 160 mass spectrbihetr'ofthe type wherein propelling electrodesare provided in the ion source. As will be apparent from the followingdescription, the invention is not limited in its utility to massspectrometry. Further, it is equally adapted to use with a massspectrometer of substantially any curvature and is independent of theparticular means employed for ionizing the sample molecules, propellingthe sample molecules from the ionization chamber and the like.

In Fig. l a part of a mass spectrometer including the analyzer tube, ionsource, and ion collector is illustrated diagrammatically as it wouldappear in plan in relation to one pole of the variable magnet. Fig. 1 isa view taken substantially on the line -i-i of Fig. 3 wherein a part ofthe mass spectrometer is shown in phantom. As shown in Fig. i the massspectrometer comprises an ionization chamber l i, an analyzer tube l2,and an ion collector i3 all of which a'r reptat low pressures during theoperation' oi the instrument. The analyzer tube i2 provided, at the endadjacent the ion collector 3, with an exit slit 1?: through which ionbeamsjare foicusfed onto the collector it by means hereinafterdescribed. U

The spectrometer is provided with a pumping system exhaust line It whichmay be connec te'd with a mercury diffusion pmnp, molecular or anyappropriate evacuating system not-shown. An inlet line 18 provides meansfor introducing a sample to be analyzed. An electron gun 'ZEl dischargesa beam of electrons through the ionizae tion chamber to ionize moleculesas they from the inlet tube it into the chamb r Pro pelling electrodes25, 21 are disposed w hin ion source so as to propel the ionsformedtherein into and through the analyzer tube. V Representation ofthis particular ion source is {0 illu's trative purposes only, therebeing many modifications thereof equally applicable to the invention.

As above described, the ions propelled source are formed in the analyzertube into diverging beams of ions of a given specificrmiass; 7

These diverging beams are successively focused on the ion collector isthrough the eXitsl it f5 by varying the magnetic field established inthe analyzertube 12 by means of the magnetshown in Figs. 2, 3 and a. Onepole {$5 of themagnet is shown in Fig. l. Theion collector is linkedwith an amplification and sensing circuit (not shown) by a lead 23sealed through the wal oi the envelope !'3. The lead 23 connects the:collector E3 to an amplifier 23 which in linked to a sensing means (notshown) such use recorder or the like.

Accelerating electrodes 2t, 21 are "connected through lead 32, 33respectively to a conventional voltage supply circuit (not shown). in 1asimilar manner the propelling electrode d the electron gun 20 areconnected to amorecr less conventional voltage supply circuit. W

The variable transverse magnetic field elmployed to focus successive ionbeams throughfth'e slit 15 in accordance with the invention sestab-:lished by means of apparatus shown inFifgs.2, 3 and 4. Referring tothese figuresthe 'yar iable shunt permanent magnet comprises asemi-cylindrical yoke til in which are two arcuate permanent magnets'42,

magnets '42, '43 are held against meenuwaustcn and 40B respectivelyofthe yoke '40.,

The magnets are provided armor inner ends with aligned arcuate polepieces 44, 45 respectively which are separated from each other by an airgap. The pole pieces are held against the inner ends of the magnets 42,43 respectively by means of bolts 46, 41 extending inwardly from the endfaces 40A, 49B of the yoke. The shape of the pole pieces is such thatthe air gap is bigger adjacent opposite ends. In this manner therelatively enlarged ends of the spectrometer tube necessary for theformation of the ion source and the collector chamber are accommodatedbetween the pole pieces. (See Fig. 3.)

The field strength across the air gap is a function of the dimensionsofthe gap and of the strength of the permanent magnets 42, 43. To permitmechanical variation of this field strength I provide a pair ofadjustable shunts 50, 5|. The two shunts 50 and 5| engage the oppositepole pieces 44 and 45 respectively and are mounted on a plurality ofshafts 53, 54 and 55. Both shunts are mounted on the same shafts and themotion of the shunts is made equal and opposite by the use of left handand right hand threads on the shafts.

Referring to Fig. 3 it Will be noted that the shaft 54 has right handthreads engaging the shunt 58 and left hand threads engaging the shunt5i. Rotation of the shaft 54 and likewise the simultaneous rotation ofthe shafts 53, 55 will cause the shunts 50, 5| either to approach eachother or retreat from each other at the same rate. Since the forces,parallel to the shafts, which are due to the action of the magneticfield on the shunts are approximately equal and opposite these forcesmerely cause longitudinal stresses in the shafts and exert practicallyno resultant force on the end bearings of the shaft. Likewise sidethrusts on the shunts are transmitted to bronze bearing inserts so thatthese side thrusts are not taken up by the several shafts. Inserts 55,51, 58 associated with the shunt 5! are shown in Fig. 4 and one suchinsert 59 associated with the shunt 50 is visible in Fig. 3. Thisarrangement makes the mechanically moving system smooth in itsoperation.

Interlocking gear means 60 connected to the several shafts 53, 54, 55permit simultaneous rotation of the shafts.

The gear means comprises three spur gears BI, 62, 83 affixed to the endsof shafts 53, 54, 55 respectively adjacent the outer face of the yokeend 40A. The several gears are all of the same size and type so thatapplication of a rotational force to one will be transmitted to theothers without speed variation. However, the direction of rotation ofthe several gears will vary. Depending on which is the drive gear, thethreading on the several shafts is arranged so that rotation thereofwill cause uniform movement of the two shunts. In Fig. 2, the centergear 6| is represented by means of the knob 54 as the drive gear. Sincegears 61 and 63 will rotate in the opposite direction than that of gear62, the threading on shafts 53, 55 is opposite to that on shaft 54.

Any means may be employed to drive the several gears. In automatic massspectrometer operations it may be desirable to employ motor drive meansso synchronized that shunt displacement may be calibrated in terms ofthe mass of the ions focused on the collector electrode. Further, eachof the mounting shafts 53, 54, 55 may be separately and synchronouslyrotated, although the simple gear arrangement shown is preferred.

The maximum field strength across the gap will be obtained when theshunts are in the open position as shown in Fig. 3. However, as theseshunts are forced toward each other by counterclockwise rotation ofshaft 54 and clockwise rotation of shafts 53, 55 the magnetic field setup between the opposing poles of the two permanent magnets 42, 43 willbe by-passed through the shunts so as to reduce the flux within the airgap.

I claim:

1. A variable shunt magnet for producing a controlled variable magneticfield in a mass spectrometer and comprising a pair of permanent magnetsarranged with one pair of opposite poles defining an air gap, apermeable yoke connecting the other pair of opposite poles, a permeablemember disposed adjacent the air gap in contact with one of the poles toshunt a portion of the flux away from the air gap and means for movingthe permeable member in relation to the air gap to vary the portion ofthe flux shunted through the member.

2. A variable shunt magnet for producing a controlled variable magneticfield in a mass spectrometer and comprising a pair of permanent magnetsarranged with one pair of opposite poles defining an air gap, apermeable yoke connecting the other pair of opposite poles, a separateperrrfeable member in contact with each of the poles defining the airgap and disposed adjacent the air gap, to shunt a portion of the fiuxaway from the air gap and means for adjusting the permeable members inrelation to the air gap to vary the portion of the fiuX shunted awayfrom the air gap.

3. A variable shunt permanent magnet for producing a controlled variablemagnetic field in a mass spectrometer and comprising a permeable yoke, apair of permanent magnets mounted in the yoke with one end of eachmagnet engaging the yoke, a separate pole piece mounted on the other endof each magnet and defining an air gap between said other ends of thetwo magnets, a separate permeable shunt engaging each pole pieceadjacent the air gap and adapted to shunt a portion of the flux awayfrom the air gap, and means for moving the two shunts with respect toeach other to vary the strength of the magnetic field in the gap.

4. Apparatus according to claim 3 wherein both-shunts are threaded on aplurality of shafts, one end of each of the shafts being journaledthrough the yoke, means for rotating all of the shafts simultaneouslyand at the same rate, each shaft being threaded in opposite directionsin the portions thereof passing through the two shunts so that rotationof the shafts causes equal and opposite linear motion of the shunts.

5. Apparatus according to claim 4 wherein the means for rotating all ofthe shafts simul-. taneously comprises a separate gear mounted on theouter end of each shaft, the several gears being identical andinterlocking and means for applying a driving force to one of the gears.

6. In a mass spectrometer comprising an ion source, an analyzer tube,means for propelling ions from the ion source through the analyzer tubeand means for producing a magnetic field in the analyzer tube transverseto the path of ion travel, the combination comprising means forproducing a controlled variable magnetic field across said analyzer tubeand including a pair of permanent magnets arranged with one pair .ofopposite poles disposed on opposite sides of the analyzer tube so thatthe analyzer tube lies 7 in the air gap between the poles, a'permeablemember disposed adjacent the air gap in contact with one of the poles toshunt a portion of the flux away from the air gap and means for movingthe permeable member in relation to the air gap to vary the portion ofthe flux shunted through the member.

7. In a mass spectrometer comprising an ion source, analyzer tube, meansfor propelling ions from the ion source through the analyzer tube andmeans for producing a magnetic field the analyzer tube transverse to thepath of ion travel, the combination comprising means for producing acontrolled variable magnetic field across said analyzer tube andincluding a pair of permanent magnets arranged with one pair of oppositepoles disposed on opposite sides of the analyzer tube so that theanalyzer tube lies in the air gap between the poles, a permeable yokeconnecting the other pair of opposite poles, and two permeable membersdisposed adjacent the air gap each being in contact with one of thepoles to shunt a portion of the flux away irom air gap and means foradjusting the permeable members in relation to the air gap to vary theportion of the flux shunted away from the air gap. l

8. In a mass spectrometer comprising an ion source, an analyzer tube,means for propelling ions from the ion source through the analyzer tubeand means for producing a magnetic field in the analyzer tube transverseto the pathof ion'travel, the combination comprising means for producinga controlled variable magnetic field across said analyzer tube andincluding a permeable yoke, a pair of permanent magnets mounted in theyoke with one end of each magnet engaging the yoke, a separate polepiece mounted on the other end of each magnet and arranged on oppositesides of the analyzer tube so that the analyzer tube lies in an .air gapbetween the pole pieces, a separate permeable shunt engaging each polepiece adjacent the air gap and adapted to shunt 'a portion of the fluxaway from the air gap, the shunts being movable with respect to eachother to vary the strength of the magnetic held in the'gap.

9. In a mass spectrometer comprising an ion source, an analyzer tube,means "for propelling the ions from the ion source through the analyzer7 tube and means for producing a magnetic field in the analyzer tubetransverse to the path of ion travel, the combination comprising meansfor producing a controlled variable magnetic field across said analyzertube and including a permeable yoke, a pair of permanent magnets mountedin the yoke with one end of each magnet engaging the yoke, a separatepole piece mounted on the other end of each magnet and arranged onopposite sides of the analyzer tube so that the analyzer tube lies in anair gap between the pole pieces, a separate permeable shunt engagingeach pole piece adjacent the air gap and adapted to shunt a portion ofthe flux away from the air gap, the two shunts being threaded on aplurality of rods journaled to the yoke, each rod extending through bothshunts and one end of each rod extending beyond the yoke, means forrotating all of the rods simultaneously and at the same rate, and eachrod being threaded in opposite directions in the portions thereofpassing through the two shunts so that rotation of the rods causes equaland opposite linear motion of the .shunts.

10. A variable shunt permanent magnet for producing a controlledvariable magnetic field in armassspectrometer and comprising a permeableyoke, a pair of permanent magnets mounted in the yoke with one end ofeach magnet engaging the yoke, a separate pole piece mounted on theother end of each magnet and defining an air gap between said'other endsof the two magnets, a separate permeable shunt mounted inproximitytozthe pole pieces adjacent to the air gap and adapted to shunta portion of the flux away from the air gap, a separate bronze bushingmounted between each pole pieceand the respective shunt with the polepiece and the shunt 'engaging the bushing, and means for moving the twoshunts with respect to each other to vary the strength of the magneticfield in the gap. HAROLD 'W. WAS-HBURN.

REFELENCES CLTED The following references are of record in the file ofthis patent:

UNITED STATES'PATENTS GTHER REFERENCES Review of Scientific Instruments,vol. 8, No. 6, June 1947, A Mass Spectrometer for Isotope and GasAnalysis by Alfred O. Nier, pages 398-411.

